Electroluminescent panels are often used in applications where only direct current (DC) power is available to energize the panels. This limitation poses unique problems because of the operating requirements of an electroluminescent panel. Electroluminescent panels must be energized with an alternating current (AC) of several hundred volts, and the life and brightness of an electroluminescent panel is determined by the voltage and frequency of the AC. In addition, the brightness of the display on an electroluminescent panel is directly proportional to the voltage and frequency of the power supplied to the panel.
In order for an electroluminescent panel to operate from a DC power source, the DC must therefore be converted to AC. The preferred conversion method is to use a DC-to-AC inverter. However, the input impedance, and more particularly, the characteristic capacitance of the electroluminescent panel must be considered in designing or selecting an inverter for this purpose. When viewed from the input terminals, an electroluminescent panel is primarily a capacitive load. Since an inverter incorporates inductors to step-up and convert the DC voltage to AC, the capacitance of the electroluminescent panel and the inductance of the inverter output interact to determine the frequency of inverter oscillation. The frequency of oscillation is approximated by the expression 1/2.pi..sqroot.LC, where L is the inductance of the inverter as viewed from the output terminals, and C the capacitance of the electroluminescent panel. As an electroluminescent panel ages, the capacitance of the panel decreases. Without compensation, this decrease in capacitance causes the intensity of the panel to dim over time. However, as should be evident from the above expression, an advantage of the interaction between the electroluminescent panel and an inverter designed to interact with the capacitance of the panel is that the decrease in capacitance causes the oscillation frequency of the inverter to increase. Since a higher frequency AC signal applied to provide power to electroluminescent panel increases its light output, the intensity of the panel remains substantially constant through the life of the panel.
While a DC-to-AC inverter is preferred as a power supply for an electroluminescent panel, there are further requirements that must be considered in providing power for this purpose. Frequently, an electroluminescent panel is used in applications where the DC voltage source is unregulated. For example, if the application is powered by batteries, the DC voltage input to the inverter will vary as the batteries age or are loaded by another part of the circuit. Also, certain types of batteries, such as lead-acid, exhibit a reduction in output voltage as they discharge. If this fluctuation in the input voltage is uncompensated, it will cause the brightness of the electroluminescent panel to vary as the AC output from the inverter varies with the changing DC.
In situations where an unregulated DC voltage source is used to provide power, additional DC regulation circuitry must be used to ensure that the electroluminescent panel maintains a constant brightness regardless of changes in the input voltage. Traditionally, as shown in the prior art circuit of FIG. 1, unregulated DC voltage 10 is regulated by a DC voltage regulator 12. This circuit uses feedback to ensure that the DC voltage output from the regulator remains constant even if the input DC voltage fluctuates. The regulated output 14 from the DC voltage regulator is then fed into DC-to-AC inverter 16. The AC output 18 of the inverter to power an electroluminescent panel 20 remains constant, because the input DC voltage is regulated to a constant level.
At least two disadvantages arise from this technique of powering an electroluminescent panel. First, the regulation of the DC voltage requires additional major circuit components, making the circuit more expensive and increasing the chance that a component failure will necessitate circuit repair. Second, the use of a two-stage process consumes additional power. Energy is lost in both the DC regulation stage and the DC-to-AC inversion stage. This inefficiency is especially of concern when batteries are used to power the electroluminescent panel, because it then becomes important to extend battery life for as long as possible.
As will be appreciated from the preceding discussion, it would be desirable to provide a power supply for an electroluminescent panel that incorporated a DC-to-AC inverter, yet simplified the means for regulating the AC output voltage to maintain a constant intensity in the light output from the panel. The DC-to-AC inverter should include the automatic compensation for the decrease in capacitance that occurs over the life of the electroluminescent panel, as is known in the prior art. However, unlike the prior art, the power supply should not require a separate DC regulator, even when provided with DC input power from an unregulated source. By using a simplified regulation method, the number of components necessary for the circuit can be reduced, thus improving circuit reliability as well as decreasing cost and increasing power efficiency.